This invention relates generally to wear and impact resistant bodies for use in industrial applications such as subterranean drilling, and cutting or machining of hard substances. More specifically, the invention provides improvements in mounting or bonding layers of superhard material to support substrates. When the superhard material is diamond the resulting bodies are generally known as polycrystalline sintered diamond compacts or PCD's.
In the following disclosure the term polycrystalline material refers to any of the superhard abrasive materials created by subjecting a mass of individual crystals to high pressure and temperature processes or to chemical vapor deposition processes such that intercrystalline bonding occurs. One class of these materials is generally referred to in the art as sintered diamond. Superhard abrasive materials include, but are not limited to, synthetic or natural diamond, cubic boron nitride, and wurtzite boron nitride, as well as combinations thereof.
These hard polycrystalline materials have been long recognized for their usefulness in cutting and drilling applications. Nevertheless, a cutting or drilling tool made entirely of polycrystalline materials is neither desirable nor practicable because the superhard polycrystalline material is relatively expensive and has relatively low impact resistance due to the high modulus of elasticity of its individual crystals. It is desirable to laminate polycrystalline materials to more impact resistant substrates.
It has long been known that polycrystalline materials can be bonded to a metallic substrate forming a compact, as shown in U.S. Pat. No. 3,745,623. This is often accomplished by sintering the polycrystalline material directly onto a precemented substrate of tungsten carbide by means of high pressure and temperature. This bonding can be accomplished with the same high pressure and temperature cycles used to create the polycrystalline material from separate crystals. An advantage of high temperature and pressure cycling in which the polycrystalline material is created by sintering and simultaneously bonding to the substrate, is that the catalyst/binder, such as cobalt, from the substrate "sweeps" through the polycrystalline material during the process effectively catalyzing the sintering process.
The substrate is bonded to the polycrystalline material under temperature conditions in excess of about 1,300.degree. C. Because of the differences in the coefficients of thermal expansion of the materials, when the compact cools, the substrate shrinks more than the polycrystalline material layer. This can create stress at the transition layer between the substrate and the polycrystalline material which can reduce the effective strength of the bond. Obviously, if the bond between the polycrystalline material and the substrate fails, the utility of the compact is lost. Such a failure may necessitate re-tooling, and thus added expense, especially in the case of deep-well and off-shore drilling applications.
Stress between the substrate and the polycrystalline material may cause fractures in the polycrystalline material, or delamination from the substrate during cooling, during attachment to a tool, or during use. In-use failures are often brought about by impact forces that release stress in the form of fractures in the compact. Ultimately, fractures lead to fracturing of the polycrystalline material, separation or delamination of the polycrystalline material from the substrate material, as well as fracture of the substrate. All of the failure modes are likely to lead to instability, and, ultimately, complete failure of the compact.
A number of configurations have been proposed to overcome the problems of stress in the compact due to thermal expansion. Some configurations suggest the use of three dimensional surface irregularities. These configurations, however have failed to suggest a way to prevent the concentration of residual stress on the critical points such as the intersections of planes.
Other configurations, particularly the configurations disclosed in U.S. Pat. No. 4,604,106, suggest that pieces of substrate material be mixed with the polycrystalline material near the transition layer prior to high pressure and temperature cycles. This is supposedly done to try to suspend the consequences of a single transitional plane. In this configuration, cobalt mixed with the polycrystalline material prevents cobalt from the substrate from cleanly sweeping impurities out of the polycrystalline material during high pressure and temperature cycles. The remaining impurities cause weak spots that can cause the part to fail.